1. Field of the Invention
The present invention relates, generally, to communication systems, devices, and processes which use radio frequency (RF) transmitters, and, in particular embodiments, to such systems, devices, and processes which minimize RF transmitter size and power requirements by producing a low noise modulated signal that eliminates the need for post-amplification transmit band filtering.
2. Description of Related Art
It has become increasingly important to minimize the size, weightand power consumption of various electronic devices, especially personal communication devices such as cellular telephones, personal pagers, cordless telephones, and the like. One way to minimize such characteristics is to minimize. the number of components and functions required in the electronic device. However, personal communication devices such as cellular telephones often require complex circuitry with a number of power-inefficient components for performing particular functions.
Upconversion and RF transmission are two such functions that typically require complex and power-inefficient circuitry. Upconversion and RF transmission are the processes of transforming a modulated information signal into an RF signal, filtering it, and amplifying it for transmission to receiving devices such as cell sites in a cellular network, and are typically performed in an RF transmitter. A conventional RF transmitter 62 is illustrated in FIG. 1, where a mixer 66 coupled to a modulator (not shown in FIG. 1) and a main synthesizer 68 receives and mixes a modulated intermediate frequency (IF) signal 64 produced by the modulator with a main synthesizer frequency 70 produced by the main synthesizer 68. The output of the mixer 66 is an unamplified modulated carrier 72. A transmit band small-signal filter 74 coupled to the mixer 66 receives and filters the unamplified modulated carrier 72 to produce a filtered unamplified modulated carrier 76. A power amplifier 78 coupled to the transmit band small-signal filter 74 receives and amplifies the filtered unamplified modulated carrier 76. The output of the power amplifier 78 is an amplified modulated carrier 80. A transmit band large-signal filter 82 coupled to the power amplifier 78 and duplexed with a receive band filter 128 receives and filters the amplified modulated carrier 80 to produce a filtered amplified modulated carrier 100. A wideband harmonic low pass filter (LPF) 84 coupled to the transmit band large-signal filter 82 receives and further filters the filtered amplified modulated carrier 100 to suppress harmonics of the transmit signal generated by the power amplifier 78. The output of the harmonic LPF 84 is a transmit signal 102. An antenna 86 coupled to the harmonic LPF 84 receives and transmits the transmit signal 102 toireceiverunits (not shown in FIG. 1).
One requirement of any RF transmitter is to produce a very low noise output to minimize the disruption to nearby receive channels. For example, in the Global System for Mobile (GSM) communication standard, the European standard for digital cellular systems operating in the 900 MHz band, frequency bands are allocated such that a mobile subscriber unit will transmit signals over a transmit band of between 890 and 915 MHz and will receive signals over a receive band of between 935 to 960 MHz. The transmit band is broken up into 125 channels, each channel separated by 200 kHz. If, as illustrated in FIG. 6, a user is transmitting at the very highest channel, 915 MHz, there will be a spike 104 at the 915 MHz carrier frequency, tapering off on either side but with measurable signal even into the 935 MHz region. Signals more than 100 kHz from the 915 MHz carrier frequency represent noise, or unwanted transmitted power. Transmitted noise extending into the designated receive band above 935 MHz is called the receive band noise 106 of the transmitter, which may interfere with the reception of other nearby mobile subscriber units. It is therefore desirable for an RF transmitter to generate very low levels of receive band noise 106 to minimize the disruption to nearby receive channels. However, RF transmitters in conventional GSM cellular telephones produce a significant amount of receive band noise that must be filtered at the output of the power amplifier.
For purposes of illustration only, the following discussion will focus on an RF transmitter in a conventional GSM cellular telephone receiving a modulated signal with a 200 kHz bandwidth at an IF of 250 MHz and transmitting at the highest channel, 915 MHz, with a receive band noise rejection requirement ofxe2x88x92164 dBc/Hz at 935 MHz, and having other intermediate component characteristics specified below. Referring again to FIG. 1, in a conventional GSM RF transmitter 62, a modulated IF signal 64 at 250 MHz having a flat but high noise floor ofxe2x88x92135 dBc/Hz (reference character 108) as illustrated in FIG. 7 is applied to the mixer 66 along with the main synthesizer frequency 70 (having a noise floor ofxe2x88x92150 dBc/Hz), which is variable to facilitate tuning to different channels. Generally, the main synthesizer 68 is designed to produce a main synthesizer frequency 70 equivalent to the carrier frequency plus the IF. In the example under discussion, the channel frequency is 915 MHz and the IF is 250 MHz, so the main synthesizer 68 will produce a main synthesizer frequency 70 of 1165 MHz.
The output of the mixer 66 is the unamplified modulated carrier 72, which retains the noise floor (reference character 108) of the modulated IF signal 64 as illustrated in FIG. 8. The unamplified modulated carrier 72 enters the transmit band small-signal filter 74 (having a bandwidth of 890 MHz to 915 MHz and attenuation of 20 dB at 935 MHz in this example), where the higher frequency receive band noise 106 is significantly reduced, resulting in the filtered unamplified modulated carrier 76 illustrated in FIG. 9. However, when the filtered unamplified modulated carrier 76 passes through the power amplifier 78 (having output power of+35.5 dBm and a third-order intermodulation product IM3 ofxe2x88x929 dBc in this example), a significant amount of the receive band noise 106 returns in the amplified modulated carrier 80 due to the nonlinearity of the power amplifier 78, as illustrated in FIG. 10. Generally, if unwanted signals are present on one side of the carrier (such as the noise floor 108 in FIG. 9), the nonlinear power amplifier 78 will create a mirror image of those unwanted signals on the other side, albeit at a lower level as determined by the third-order intermodulation product. Thus, in conventional approaches, no amount of filtering by the transmit band small-signal filter 74 on the input side of the power amplifier 78 will eliminate the need for a transmit band large-signal filter 82 on the output side.
As a result, filtering of receive band noise 106 is performed at the end of the transmit chain, after the power amplifier 78 but before the antenna 86. In the GSM example under discussion, the transmit band large-signal filter 82 comprises a bandpass filter with a pass band of about 890 to 915 MHz and attenuation of 20 dB at 935 MHz to pass the entire transmit band, reject the entire receive band, and push down the receive band noise 106 as illustrated in FIG. 1. However, the transmit band large-signal filter 82 has significant insertion loss, typically 2.0 dB, that may eliminate as much as half of the transmitted power. Because GSM cellular telephones can produce a selectable amount of output power, but no more than two watts, the RF transmitter 62 may therefore need to generate as much as four watts of transmit power in order to produce an actual post-filter output of two watts. In this example, significant battery power is wasted in supplying four watts of power to the power amplifier 78 to produce a two watt transmit signal 102.
An alternative to the conventional RF transmitter 62 is a translation loop transmitter 62xe2x80x2 illustrated in FIG. 2. In this approach, the mixer 66 and transmit band small-signal filter 74 of FIG. 1 are replaced by a translation loop 112 capable of producing a very low noise signal. The translation loop 112 includes a voltage controlled oscillator (VCO) 94, a mixer 114 for determining the difference between the frequency of the VCO 94 and a main synthesizer frequency 70 from main synthesizer 68, a low-pass filter 96 for filtering the output of the mixer 114, a phase detector 88 for determining the phase difference between the mixer output and the modulated IF signal 64, a charge pump 90 for sourcing or sinking current as determined by the phase difference output of the phase detector 88, and a loop filter 92 for integrating current pulses from the charge pump 90 and providing a control voltage to the VCO 94. Low receive band noise is achieved not by post-power amplifier filtering, but by generating minimal noise prior to amplification, resulting in minimal unwanted signals for the nonlinear power amplifier to amplify and reflect.
In the translation loop approach of FIG. 2, the translation loop 112 translates the modulated IF signal 64 to the carrier frequency of 915 MHz. The translation loop 112 is designed to have a bandwidth of about one MHz, so any disturbances outside this bandwidth will be rejected. Thus, the translation loop 112 is essentially a 1 MHz wide filter, tunable to the selected carrier frequency. The key to the translation loop approach is the VCO 94, which must be chosen to meet the desired noise requirements. By utilizing a VCO 94 with a noise floor ofxe2x88x92164 dBc/Hz (which meets the desired noise requirements), noise surrounding the carrier frequency is greatly reduced, and there is very little noise to be reflected when the translation loop output 116, illustrated in FIG. 12, is amplified by the power amplifier 78 (with output power of+34.0 dBm in this example). Because of the low noise on the output of power amplifier 78 (amplified modulated carrier 80) illustrated in FIG. 13, no power-inefficient transmit band large-signal filter is needed, and thus only a low loss T/R switch 98 is needed for connecting the antenna to either the transmit or receive electronics, but not both. However, some of the power savings is lost because the low noise VCO 94 is a high power consumption device. In addition, the translation loop approach is larger, more expensive, and more complicated due to the additional components required.
Therefore, it is an object of embodiments of the present invention to provide a device, system and method for low noise RF transmission of signals that is also lower in power consumption relative to conventional systems, devices, and methods.
It is a further object of embodiments of the invention to provide a system, device, and method for low noise RF transmission of signals that is also lower in cost, size, and complexity relative to conventional systems and methods due to its avoidance of translation loop circuitry.
These and other objects are accomplished according to a transmitting device that receives a modulated signal having a modulation bandwidth and an intermediate center frequency and generates a modulated transmit band signal at a carrier frequency. The transmitting device includes a first filter that receives the modulated signal and passes frequencies within the modulation bandwidth, producing a low noise filtered modulated signal, and a low noise frequency source that generates a low noise main synthesizer frequency. The filtered modulated signal and the main synthesizer frequency are then fed into a mixer that mixes the two signals (produces either the sum or difference). The output is a low noise modulated transmit band signal that may be amplified to transmit power levels without the need for power-inefficient post-amplification filtering.
These and other objects, features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims.